This invention relates to skeletal isomerization of olefins, i.e., to the reorientation of the molecular structure in respect to the formation or elimination of side chains. This invention more particularly relates to the conversion of unbranched olefins into branched olefins having the same number of carbon atoms.
Skeletal isomerization of olefins is known to be accomplished by contacting unbranched or lightly branched olefins with acidic catalysts at elevated temperatures. The process is generally applicable to the isomerization of olefins having about 4 to about 20 carbon atoms and is especially applicable to olefins having about 4 to about 10 carbon atoms per molecule. The process may be used to form isobutene from normal butenes, methyl pentenes and dimethyl butenes from normal hexenes, and so forth.
Known skeletal isomerization catalysts include aluminas and halogenated aluminas, particularly F- or Cl-promoted aluminas. See Choudary, V. R., "Fluorine Promoted Catalysts: Activity and Surface Properties", Ind. Eng. Chem., Prod. Res. Dev., 16(1), pp. 12-22 (1977) and U.S. Pat. Nos. 4,400,574 and 4,404,417. U.S. Pat. No. 3,558,734 teaches catalytic skeletal isomerization of hydrogen-diluted olefins in the presence of a halogen compound and/or water. The specification stresses the use of Cl- or F-impregnated alumina catalysts.
U.S. Pat. No. 3,663,453 teaches skeletal isomerization of olefins by contact with alumina promoted by a zirconyl halide. Zirconyl chloride is preferred, the order of increasing effectiveness of the halides being iodide, fluoride, bromide and chloride. The catalysts are prepared by impregnation techniques. The olefin feed preferably contains a halogen compound to maintain catalyst activity.
An object of this invention is an improved process for the skeletal isomerization of olefins, especially for the isomerization of n-butenes to form isobutene. A more specific object is an easily prepared, stable, active and selective isomerization catalyst and process for skeletal isomerization of olefins. Other objects and advantages of the invention will be apparent from the following description.